For transmission of digital video, audio and data signals, it is possible to use either a landline transmission via an antenna in the air, a satellite connection, a cable TV network, a telephone or telecommunication network, or a light cable network, through which digital information is transmitted to a large number of receivers. An advantage of the transmission of digital signals over analog transmission is the fact that the same transmission channel can be used for the transmission of signals from several different program sources. This can be implemented for example in a way that the signals are divided by a method known as such to packets which are transmitted on the transmission channel. Packets formed of different program sources are broadcast in a time-interlaced manner, wherein a packet comprises advantageously at name field bearing information on which program source said packet belongs to, and a data field having the information to be broadcast.
Before analog signals can be broadcast in packet form, the analog signals must first be converted to digital signals at the sending end, wherein the digital signal formed can be arranged in packets to be sent to the transmission channel. At the receiving end, the analog signals converted to digital signals are converted back to analog form.
One important application of transmission of digital signals in packet form is the digital television system, in which several programs are transmitted simultaneously in one broadcasting channel. Thus the broadcasting channel carries packets containing video, audio and data information of different programs in turns. One example is the MPEG-2 (Motion Picture Experts Group) which defines the packeting of information as well as a generic standard for the video and audio compression method, making it possible to broadcast a television image with fewer bits than by digitizing the television image directly to bits. One video data packet usually covers image information from several macroblocks containing 16.times.16 pixels. FIG. 1 shows a data transmission system, in which video, audio and data signals formed by different program sources are transmitted in the channel to the receiver. At the broadcasting end, packets are formed from different types of information, such as video, audio and data, which are integrated into program packets, for example by a multiplexer 2. Further information on the program, such as service information (SI) identifying the program, is added to the program packets in a second multiplexer 3. FIG. 1 shows the packet-stream forming devices 1a, 1b, 1c, 2, 3 for only one program, but at the transmitting end, several different programs can be combined to the same data transmission stream. The system of FIG. 1 contains a program multiplexer 4 for integrating packets from different program sources advantageously to a data transmission stream in packet form for broadcasting further. Upon broadcasting, it is still possible to encrypt all or some packets using an encryption device 5, after which the data transmission stream is channel-coded with a channel-coder 6. The channel-coder 6 converts the data transmission stream to a signal which is suitable for the data transmission channel used at the time, as is known. The data transmission channel 7 is for example a satellite connection, a cable connection or a landline link connection.
At the receiving end, the converse operations are performed in relation to the sending, i.e. channel decoding, deciphering, separation of Si information, and demultiplexing of program packets, wherein the packets belonging to a program to be watched Eat a time are separated as one signal from the data transmission stream, where in the video packets, audio packets and data packets are separated from the signal of the program to be received.
For watching digital video signals, receiving devices have been developed, namely so-called Set-Top-Box (STB) devices. In these STB devices, the received digital video signal is demodulated and error-correction is conducted for elimination of possible errors that have interfered the signal on the transmission channel. Further, from the stream of packets, those packets are selected which belong to the program that is intended to be watched, and possible encryption is deciphered, provided that the viewer has the right for encryption deciphering. After this, the deciphered signal is decoded, that is, the reverse operation is performed in relation to the coding at the transmission stage, for example coding according to the MPEG-2 algorithm. Decoding is conducted on both audio and video signals, wherein the decoder advantageously makes both an audio and a video signal. The decoded signals are at this stage still in digital form, the audio signal is still directed to the digital-to-analog converter and the video signal is directed e.g. to a PAL converter or a NTSC converter according to the system used for transmitting the color information of the video image. The above-mentioned PAL and NTSC color systems are the most common ones, but also other color coding systems are known. The block chart of FIG. 1 is a reduced illustration of the operational blocks of a STB device: the channel coder 8, the encryption deciphering means 9, a demultiplexer 10 for separation of service information from the packets, a demultiplexer 11 for program packets, as well as means 12a, 12b, 12c for separation of program packets and formation of video, audio and data signals corresponding to the broadcast signals. Further, the STB device can include means for storing service information.
The audio and video signals generated by the STB device are conveyed to a conventional television set, particularly to the video or SCART connection of the television, wherein the received information can be watched on the television screen.
At the transmission end of digital television systems, the transmission servers form digital packets of several programs to be broadcast, and these packets are transmitted to the transmission channel, such as a satellite 16a, 16b (FIG. 1b). The programs to be broadcast may include for example commercial videos, music videos, movies, sports events, current affairs programs, and news. The STB device is capable of selecting from the incoming data transmission stream the program that the viewer wishes to watch. Thus the viewer must give the device the identification of the program in question, or the STB device can make a list of the identifications of those programs that are included in the data transmission stream to be received, for example in the signal from the satellite to which a receiving antenna 21 is directed.
FIG. 2a shows an example of giving the title of a program in text form. The first field shows the identification of the protocol to be used, such as digital video broadcasting (DVB) or hypertext transfer protocol (HTTP). The protocol identifier is followed by a protocol separator, with which the protocol field is separated from the next field. The next field is a service name field, which is again followed by a field separator. The third field is the service provider name field, which is separated by a field separator from the fourth field, i.e. the network name field. The network name field is finally followed by a name separator.
One advantage of the above-mentioned naming practice is that when the physical properties of the transmission network are changed, for example the broadcasting frequency or the location of a satellite is changed, this change will have no effect on the user. On the other hand, the STB device will also require knowledge on where the said program broadcasting can be received, wherein the STB device must also be transmitted the physical parameters of the broadcasting. FIG. 2b is an exemplified illustration on the presentation of one text-form service information item in numerical form, also including information on the physical parameters. In case the viewer would like to watch this program, he or she should enter the numerical data to the STB device, or this data should be stored using another known method. This involves the disadvantage that any changes in the physical parameters should be updated to the STB device. Such numerical strings are difficult to remember, and their input will easily involve writing errors which are not even necessarily noticed, in which case it is not possible to watch the desired program.
STB devices are known which have browsing properties, wherein the STB device forms advantageously a list to be displayed of the programs which are transmitted in the data transmission stream received by the STB device at a time. The viewer can then select from the list the program he or she wishes to watch. However, this system involves the disadvantage that the list of programs is limited to the programs in the data transmission stream received at the time. The viewer may still have access to the address data of a program which is broadcast via another data transmission stream, maybe via a different type of data transmission channel. Thus the viewer must also enter into the STB device also the parameters of the data transmission channel which are not necessarily known to the viewer.